Conventionally, an RFID (Radio Frequency Identification) system, which is configured of a device holding an inherent ID and a device reading off it, has a problem that an erroneous detection and a detection omission occur and resultantly the detection of the ID becomes unstable due to influences such as an antenna directivity of an ID holding device and an ID reading device, an disorder of a radio wave environment caused by a multipath etc., a change in an antenna characteristic caused by a person or a thing to which the ID holding device has been affixed, and radio wave noise from a personal computer located in the adjacent.
So as to reduce the erroneous detection and the detection omission in the RFID system and stabilize the detection, in Patent document 1, a positional relation between the ID holding device and the ID reading device is positively changed, and simultaneously therewith, a plurality of the ID holding devices are prevented from being positionally superposed onto each other, thereby allowing the status in which the radio wave necessary for reading is obtained even in an instant to be created.
Further, in Patent document 2, so as to put a limit to a range in which the detection is made by the ID reading device, providing a shielding plate allows an arrival scope of the radio wave to be limited.
Further, in Patent document 3 and Patent document 4, accommodating the ID holding device in a shell that can be maintained at any time in a state directed to a direction suitable for the reading-off allows the detection to be stabilized.
Further, in Patent document 5, Patent document 6, Patent document 7, Patent document 8, or the like, making an improvement to the device itself of the RFID system, for example, a control of a transmission power, a power supply management, a circuit design for improving a signal-to-noise ratio, and a contrivance of a transmission/reception antenna structure allows the detection to be stabilized.
Further, as a general process of stabilizing the detection, there exists the timeout process of determining that the detection target has left a detection area when the detection target was not be able to be read off for a constant time for a history of a read result, the average value computing process of obtaining an average radio wave intensity in a constant time or a radio ratio in a constant time for a history of a read result, the thresholding process of determining that the detection target has left the detection area when the radio wave intensity or the radio ratio in a constant time has fallen under a constant value, or the like.    Patent document 1: JP-P2002-329174A    Patent document 2: JP-P2004-059320A    Patent document 3: JP-P2003-058843A    Patent document 4: JP-P2002-259932A    Patent document 5: JP-P2002-170082A    Patent document 6: JP-P2001-307052A    Patent document 7: JP-P1999-282977A    Patent document 8: JP-P1998-242893A